From: Jet Propulsion Laboratory
Posted: Friday, December 2, 2005
By Lucy McFadden
In the past month, the science team has continued with its data analysis. Many people don't realize the detailed computations that are required to convert a picture made up of raw data numbers (DN) returned from space, into an image containing numbers of physical meaning. This process is called calibration.
It is carried out by observing stars of known light output, or radiance, and scaling the value of that output to the known quantity of radiation produced by the star. This is like synchronizing your watch, so that everyone has the same time reference. In this case the reference is to a scale of energy output. We have updated our calibration numbers using the most recent data taken just before and after impact. We have made improvements in subtraction of the background signal that exists in every electronic detector so that we can analyze the signal from the comet and not the noise from the camera's detector. With the known value of the star expressed in units of energy, we then determine the energy released from the comet. Ken Klaasen and other team members have been working hard on this.
Another effort that has taken significant time has been processing images to find evidence of the crater formed by the impact. Our experts in deconvolution and image processing have taken on the task of image enhancement to find the crater. It is apparent that the crater cannot be seen with certainty through all the dust that was ejected from the impact. We measure the width of the shadow cast by the ejecta plume to set a maximum value for the crater diameter. The crater cannot be larger than the width of the shadow cast.
In analyzing the infrared spectrum, one asks does this spectrum represent the nucleus or ejecta? The team is working with engineers to get the best understanding of where the IR spectrometer aboard the flyby spacecraft is pointing. In some cases, the IR spectrum was very different from one second to the next. Was this due to a change in the pointing of the spacecraft, or a change in the nature of the material observed? To be sure, we had to verify the spacecraft pointing using telemetry. We needed engineering verification of what the IR spectrometer was "seeing" so that we didn't interpret a feature as something on the nucleus when in fact, the spectrometer may have been pointing at the expanding ejecta cloud. Engineers have confirmed our understanding of the spectrometer's pointing. The MRI and HRI cameras allow us to associate the spectra with position on the nucleus, or off the nucleus, as the case may be - see figure 1.
[Figure 1: Follow the dotted line] Images of impact taken with MRI. The blue dotted line is the position of the spectrometer's slit. At each point along the slit, a spectrum exists that contains information about the constituents in the spectrometer's field of view. [larger view]
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